Various types of hydraulic drive systems are known for material handling implements such as tractors, loaders, and like equipment. One type of hydraulic drive system frequently used is referred to as a closed loop hydrostatic transmission. This type of system includes a reversible flow, variable displacement hydraulic fluid pump hydraulically joined in a closed loop with either a fixed or variable displacement hydraulic motor. An internal combustion engine is typically provided for driving the variable displacement pump. Control of pressurized hydraulic fluid within the closed loop is provided by a manually operated linkage which operates the variable displacement pump to control the direction and rate of hydraulic fluid flow within the closed loop. In this way, the hydraulic motor of the hydrostatic transmission may be selectively operated in forward and reverse directions at varying speeds.
The hydraulic motor of the closed transmission loop is connected through a suitable drive train with the drive means of the implement, such as wheels or a crawler track. Typically, the implement transmission includes a pair of closed hydrostatic transmission loops, each having a variable displacement pump hydraulically joined with a hydraulic motor. The motor of each loop is connected with wheels or a crawler track on one side of the implement or the other. Because the closed loops may be independently operated, this type of arrangement permits the operator of the implement to not only drive the implement in forward and reverse directions, but permits the implement to be maneuvered by selective independent control of each of the closed transmission loops. Drive systems of this description are typically employed in so-called skid-steer loaders, crawler tractors, and other similar implements where independent control of drive means on opposite sides of the implement permit the implement to be maneuvered as desired.
As with most hydraulic systems, optimum performance of closed loop hydrostatic transmissions relies, in part, upon maintaining the hydraulic fluid within the system within a certain specified temperature range. If the temperature of the hydraulic fluid within the system is higher than the desired level, the lubricating characteristics of the fluid may be impaired, and breakdown of the fluid into sludge and other contaminants may result. Additionally, excessive heating of the hydraulic fluid within the transmission system may result in premature failure of seals and other components within the system, while resultant loss of viscosity of the fluid at higher temperatues reduces the efficiency of the hydraulic fluid pumps in the system.
Since fluid friction resulting from flow of hydraulic fluid within the closed loops of the hydrostatic drive system creates heat which increases the temperature of the fluid within the system, it is necessary that some arrangement be provided so that the temperature of the fluid within the loops is maintained within the desired specified range. While some of the heat may be naturally dissipated from the system, most hydrostatic transmission systems include an arrangement for replenishing the hydraulic fluid in the closed loops with relatively cooler make-up or charging fluid.
A typical closed loop hydrostatic transmission system includes a hydraulic fluid charging pump which is provided to introduce hydraulic fluid to the closed loops of the transmission from a hydraulic fluid reservoir. The primary purpose of the charging pump is to maintain positive pressure or head on the inlet or low pressure (suction) side of the variable displacement pumps of the closed transmission loops. This pressure is necessary to ensure that the piston shoes of the typical variable displacement pump are maintained tightly against the tiltable swash plate of this type of pump; damage to the piston shoes would otherwise occur. A further function performed by the charging pump of the transmission system is to replenish hydraulic fluid in the closed loop with the relatively cool hydraulic fluid drawn from the fluid reservoir of the system.
The introduction of cooler charging fluid to the closed loops of the hydrostatic transmission is in part accommodated by the provision of case drains in each of the variable displacement pumps of the transmission loops. The pump case drains, which permit proper operation of the reversible variable displacement pumps, provide "leakage" of hydraulic fluid from the closed loops of the transmission. The charging system replenishes the fluid in the closed loops which is lost in this manner, directing relatively cooler hydraulic fluid to the low pressure sides of the transmission loops through check valves. Fluid from the case drains of the hydraulic pumps is returned to the fluid reservoir of the system so that the excess heat in the fluid may be dissipated.
Because fluid lost from the closed loops of the transmission through the pump case drains may be insufficient to permit proper replenishment of the fluid in the loops with cooler charging fluid, various arrangements are known for providing a controlled bleed of hydraulic fluid from the closed loop lines of the transmission. Typically, a hydraulic fluid bleed valve is teed into each of the closed loops of the transmission, generally bleeding a more or less constant amount of hydraulic fluid from each of the loops. The fluid bled from the loops in this manner is replenished with cooler charging fluid from the charging system so that the temperature of the fluid within the loops is maintained within the specified range.
For the most part, this type of closed loop hydrostatic transmission, including pump case drains and loop fluid bleeds, is effective in maintaining the temperature of the fluid as desired. For instance, when the closed loops are operated at relatively high pressures, such as during work operations, a relatively greater amount of hot fluid is forced out of the case drains of the hydraulic pumps of the loops, and is replaced with cooler charging fluid by the charge system. While fluid overheating is usually not a problem under these operating conditions, the hydraulic pump of the transmission charging system must be of sufficient displacement to replenish the fluid lost from the loops through the pump case drains and fluid bleeds, even though the amount of fluid lost from the loops may be in excess of that needed for proper fluid temperature control.
In contrast, overheating of hydraulic fluid in the closed loops is a common problem in those situations where there is relatively high fluid flow or displacement at relatively lower pressure. This will typically be the situation during "roading" of the implement, i.e., when the implement is operated at relatively higher speeds with little or no load (such as driving between job sites, etc.). It will be appreciated that in these situations, because the system is operating at a relatively lower pressure, a relatively lesser amount of hot fluid from the loops is forced out of the pump case drains. Because typical bleed systems for bleeding fluid from the closed loops do not differentiate between conditions of relatively high and low fluid pressure within the loops, bleeding off a more or less constant amount of fluid, roading of the implement as described can result in excessive elevation of fluid temperature within the loops. This is because the relatively lesser amount of hot fluid flow from the loops through the pump case drains, together with the generally constant flow through the bleed valves, does not permit sufficient cooler charging fluid to be introduced into the loops to maintain the fluid temperature in the loops within the desired range. As described, operation of the hydrostatic transmission with the hydraulic fluid at temperatures beyond the specified range is detrimental to the components of the system.
Thus, a hydraulic fluid bleed arrangement for a closed loop hydrostatic transmission which responsively varies the amount of fluid bled from the loops acts to overcome the problems of fluid overheating which may be encountered with typical fluid bleed arrangements and improves the reliability and performance of these type of drive systems.